Received: 14 May 2018 Revised: 22 December 2018 Accepted: 27 December 2018 DOI: 10.1111/pce.13516 ORIGINAL ARTICLE Anatomical changes with needle length are correlated with leaf structural and physiological traits across five Pinus species Na Wang1,2 | Sari Palmroth2 | Christopher A. Maier3 | Jean‐Christophe Domec2,4 | Ram Oren2,5 1 School of Forestry, Northeast Forestry University, Harbin 150040, China Abstract 2 Nicholas School of the Environment, Duke The genus Pinus has wide geographical range and includes species that are the most University, Durham, NC 27708, USA economically valued among forest trees worldwide. Pine needle length varies greatly 3 Southern Research Station, USDA Forest among species, but the effects of needle length on anatomy, function, and coordina- Service, Durham, NC 27709, USA 4 Bordeaux Sciences Agro, UMR 1391 INRA- tion and trade‐offs among traits are poorly understood. We examined variation in leaf ISPA, 33175 Gradignan Cedex, France morphological, anatomical, mechanical, chemical, and physiological characteristics 5 Department of Forest Sciences, University of among five southern pine species: Pinus echinata, Pinus elliottii, Pinus palustris, Pinus Helsinki, Helsinki, Finland Correspondence taeda, and Pinus virginiana. We found that increasing needle length contributed to a Sari Palmroth, Nicholas School of the trade‐off between the relative fractions of support versus photosynthetic tissue Environment, Duke University, Durham, NC 27708, USA. (mesophyll) across species. From the shortest (7 cm) to the longest (36 cm) needles, Email: [email protected] mechanical tissue fraction increased by 50%, whereas needle dry density decreased Na Wang, School of Forestry, Northeast by 21%, revealing multiple adjustments to a greater need for mechanical support in Forestry University, Harbin 150040, China. Email: [email protected] longer needles. We also found a fourfold increase in leaf hydraulic conductance over Funding information the range of needle length across species, associated with weaker upward trends in National Science Foundation, Grant/Award stomatal conductance and photosynthetic capacity. Our results suggest that the leaf Number: NSF‐IOS‐1754893; Erkko Visiting Professor Programme of the Jane and Aatos size strongly influences their anatomical traits, which, in turn, are reflected in leaf Erkko 375th Anniversary Fund, through the mechanical support and physiological capacity. University of Helsinki KEYWORDS conductance, leaf structure, leaf traits, nitrogen, photosynthesis, pine, plant hydraulics, stomata, water relations, xylem transport 1 | INTRODUCTION the leaf boundary‐layer conductance and thus water loss rate and leaf temperature (Yates, Verboom, Rebelo, & Cramer, 2010). Furthermore, Leaf size exhibits great plasticity, both along environmental gradients leaf size affects plant productivity and adaptive capacity to changes in and within communities, varying by over 100,000‐fold among species its environment (Pickup, Westoby, & Basden, 2005; Westoby, Falster, worldwide (Milla & Reich, 2007; Wright et al., 2017). Corner's Moles, Vesk, & Wright, 2002; Wright, Falster, Pickup, & Westoby, hypothesis states that, compared with tree species with smaller leaves, 2006). A better understanding of constraints on the structural design species with large leaves have greater proportions of their shoot bio- of leaves, and the resulting coordination or trade‐offs among leaf traits mass invested in foliage and hold more widely spaced leaves on shoots across species, is key to understanding variation of plant growth rates (Corner, 1949). Thus, species with large leaves tend to experience less and adaptive and competitive capabilities. self‐shading within a shoot (Falster & Westoby, 2003). Leaf size The genus Pinus has a wide geographical range and includes affects, or is related to, many functional traits. Combined with leaf species that are the most economically valued among forest trees organization on shoots, leaf size affects light interception efficiency worldwide (Wear & Greis, 2002). Among conifers, pines species show and the net carbon assimilation capacity of a plant, in part by affecting the largest range of needle length, yet relatively little is known about 1690 © 2019 John Wiley & Sons Ltd wileyonlinelibrary.com/journal/pce Plant Cell Environ. 2019;42:1690–1704. WANG ET AL. 1691 the influence of needle length on needle structure and function and needle length to irradiance on needle surfaces. That is, in a given light how variations in anatomical traits affect mechanical and physiological environment, mutual shading among needles of a shoot (inversely traits. More is known about how needle length and organization affect related to STAR) STAR appears to reach a minimum at intermediate light distribution on needle surfaces (Niinemets, Tobias, Cescatti, & needle lengths, providing the highest mean irradiance on needle Sparrow, 2006; Stenberg, Palmroth, Bond, Sprugel, & Smolander, area relative to shoots with either shorter or longer needles. If light 2001; Thérézien, Palmroth, Brady, & Oren, 2007). The amount of light availability was the main driver of photosynthetic capacity and related intercepted by unit of leaf area depends on its location relative to leaf traits, these traits should show a similar pattern with needle length other shading elements in the canopy. In coniferous canopies, three‐ among species. dimensional foliage elements are grouped into shoots, that is, (semi‐) However, mechanical traits control the physical response of annual growth units, often considered the basic functional units. The leaves to environmental forces, with physiological and ecological con- mean light intensity, or mean irradiance, on needle surfaces of a shoot sequences (Niklas, 1999). Needles work as cantilevered mechanical depends on the amount of light reaching the shoot and on the shoot “devices,” bearing a relatively uniform load along their length. They interception efficiency. This efficiency varies with the size and orienta- must be sufficiently rigid to support their own weight mechanically tion of the shoot and the dimensions and organization of the needles against the pull of gravity yet flexible enough to bend or twist without (Niinemets, Tobias, et al., 2006; Stenberg et al., 1994). Ceteris paribus, breaking when subjected to strong and dynamic external forces, such the mean shoot silhouette‐to‐total leaf area ratio ( STAR; Stenberg as wind, snow, hail or ice formation, animal action, abrasion against et al., 1994) first increases with needle length, reaching a maximum other canopy elements, and falling debris (Raupach & Thom, 1981; and decreasing thereafter with further increases in length (Figure 1a, Vogel, 1989). Previous studies demonstrated that the investment in simulated using the model by Thérézien et al., 2007). This simplified support biomass (midrib and petiole mass) per unit leaf area (or leaf analysis does not account for other shoot structural characteristics, fresh mass) in larger leaves is greater than in smaller leaves (Niinemets, which vary among species, yet suggests a hypothesis that directly links Portsmuth, & Tobias, 2006). In conifers, the bending force operating on a needle is proportional to the cube of its length (Niklas, 1992, 1999); thus, the biomass or volume fraction of supporting tissues (epidemical tissue plus xylem) must scale exponentially with needle length to main- tain a given needle angle. However, among pine species, needle cross section does not increase with increasing needle length (Figure 1b, Table S1). This suggests that the extra structural support required for longer needles is not achieved simply through increased cross‐ sectional area and associated strength but by increasing the relative investment in mechanical tissues. The hydraulic costs and benefits of deploying a given leaf area as many small versus fewer large leaves are still a matter of debate (Corner, 1949; Givnish, 1984; Niinemets, Portsmuth, & Tobias, 2006; Yang, Li, & Sun, 2008). The pathway of water within leaves represents a substantial portion of the whole‐plant resistance to water flow from root to the substomatal air space (Nardini & Salleo, 2000; Sack & Holbrook, 2006). Thus, for example, correlations have been noted among leaf hydraulic conductance (Kleaf), maximum stomatal conduc- tance (maximum gs), transpiration, and photosynthetic rates across species (Brodribb, Feild, & Jordan, 2007; Sack, Cowan, Jaikumar, & Holbrook, 2003; Sack & Holbrook, 2006), with additional variation caused by resource availability (Domec et al., 2009; Sellin, Sack, Õunapuu, & Karusion, 2011). In angiosperms, Kleaf varies indepen- dently of leaf size among species because of the overwhelming effects of the great phylogenetic and developmental diversity of the vascular architecture (Sack et al., 2003; Sack & Frole, 2006). In contrast to the complex leaf vein patterns in angiosperm, pine needles have a very simple design that consists of a single middle vein (Esau, 1977; Zwieniecki, Stone, Leigh, Boyce, & Holbrook, 2006). However, whether the diversity in needle length translates to large differences in water transport capacity and, thus, in stomatal conductance and FIGURE 1 Variation in modelled spherically averaged shoot‐ gas exchange remains unclear. silhouette‐to‐total‐leaf‐area ratio (STAR; a), and needle cross‐ The hydraulic conductance of xylem, and along the flow paths sectional area (b) with needle length. The model used in (a) is from Thérézien et al. (2007). The data in (b) were